In this document I attempt to define the technical terms I use in Jonathan's Space Report and associated material (the General Catalog of Artificial Space Objects, the Deep Space Catalog, etc.).
Some of these definitions reference GCAT (the General Catalog of Artificial Space Objects, https://planet4589.org/space/gcat/
People have strong feelings about the meaning and usage of words, and I am sure some readers will disagree with my choices. Here I am expressing what *I* mean by the words in question, in particular when I use them in these JSR documents. This document is an excuse for me to indulge my tendency for pedantry to a previously unprecedented degree. I have concentrated particulary on including terms which tend to be (in my opinion) misused, or which have multiple senses. I have added extra detail in cases where everyone else is wrong and I'm right (funny how that happens...). You may find the inclusion (or exclusion) of terms to be idiosyncratic. I am happy to consider suggestions for additional terms, but I make no promises.
Words in English often have multiple senses. A word may have a narrow and broad sense (see here `satellite'). Words may be homographs or homonyms, with the same spelling but unrelated meanings (`row', `bat', `down', etc.) I attempt to be explicit about these multiple senses.
I am a dual US-Brit (My parents were UK citizens born in Belfast who moved to England; I was born in Atlanta, Georgia, moved to the UK as a baby, was brought up mainly in England but a bit in the US too, and with one toe in France.) Therefore, my idiolect can legitimately swap between UK and US spellings in the course of a single sentence - you have been warned.
With that, let's proceed - I hope someone finds this useful.
Once the rocket has lifted off, an abort is usually fatal to the rocket and may involve remote-commanded destruction of the rocket.
In human spaceflight, a launch abort will usually involve the use of a preplanned strategy to return the crew safely to Earth, for example with the use of a launch escape tower, or just by separating the spaceship into a ballistic trajectory as was done on Soyuz MS-10.
The Space Shuttle had a number of predefined abort strategies, which included:
A related concept is aerocapture, in which aerobraking is used to turn a hyperbolic orbit into a bound elliptical one.
Let the Keplerian elements (q.v.) include semi-major axis a and eccentricity e. Then we define
Hp = a (1-e) - R
Ha = a (1+e) -R
Some have pointed out that the formation of the generic words is inconsistent with those of the specific: the apsis is the end of the orbital radius, not the body center. The word `apoapsis' must be read as 'furthest apsis', while 'apogee' is 'furthest *from* the Earth'. Writers who find this inconsistency problematic advocate the uglier `apocenter' and `pericenter'. I am not one of them.
See the GCAT discussion of the Friends of Perigee and Apogee.
In a hyperbolic orbit, with e greater than 1, the semimajor axis a is negative and doesn't have the same simple geometrical interpretation. Since 1-e is also negative, the formal periapsis radius a(1-e) remains positive, and indeed the perapsis height does retain its usual interpretation as the mimimum height in the orbit. The apoapsis height, however, is negative; we treat it as a formal value without geometric meaning.
If the satellite is spin-stablized and the rocket injects the satellite at perigee of the transfer orbit with its 'top' in the direction of motion, you don't need to orient the satellite further. Just wait half an orbit with a timer, and the satellite will be at apogee with the 'top' now facing opposite the direction of motion (same direction as before in inertial space). So if you put the rocket nozzle sticking out the top of the satellite, just fire it and you'll get the right result without the need for an attitude control sensor or maneuver. That was a pretty neat trick in the late 1950s; nowadays accuracy standards are too high to rely on it.
I consider all humans who have flown above 80 km to be astronauts (see `space', sense 2.) Arguably also other primates who have done so.
An attitude maneuver changes which way the satellite is pointing as it follows its orbit; it does not change the orbit itself.
A ballistic missile weapons system many consist of some or all of the following components:
The US military has an elaborate classification of ballistic missiles based on their range. (Note that other counties may not share the same definitions).
|SRBM||Short Range Ballistic Misisle||Less than 1000 km|
|MRBM||Medium Range Ballistic Missile||1000 to 3000 km|
|IRBM||Intermediate Range Ballistic Missile||3000 to 5500 km|
|ICBM||Intercontintental Ballistic Missile||above 5500 km|
|TBM||Tactical Ballistic Missile||Less than 300 km?|
|TBM||Theatre Ballistic Missile||Less than 3500 km|
The Outer Space Treaty uses the term `celestial body' which means the same thing EXCEPT that the Earth is not considered a `celestial body' by the OST, but I do consider it to be one.
The term `primary' is often used in astronomy as a synonym for `central body', particulary in the context of multiple-star systems.
A special case is when objects are in the vicinity of one of the Lagrange Points (q.v.) of two bodies. The motion of the objects can then be thought of as being `orbits' around the Lagrange Point, considered as a fictitious body, instead of around an actual massive body. Such orbits are strongly non-Keplerian.
Note the common acronyms BECO, SECO, MECO: Booster/Sustainer (or Second Stage)/Main Engine Cutoff.
Note that the term `satellite bus' means something different to spacecraft electrical engineers: the electronics and wiring system that sends information and power around the satellite. I never use the term in this sense.
For science satellites, the POC is sometimes called a Science Operations Center (confusingly, SOC).
The sky is divided into 88 constellations, whose boundaries were fixed in 1926 by the International Astronomical Union.
Orbital debris is also informally referred to as `space junk'.
A 2020 web article from the Space Data Association described the destruction of the FY-1C and USA 193 satellties with antisatellite weapons as decommissioning them. However, this kind of `decommissioning with extreme prejudice' is not normally included within the concept.
The advent of the Starlink constellation has introduced a qualitatively new kind of deorbit. The Starlink satellites are retired by continuously lowering their orbit with electric propulsion. Reentry occurs in a way similar to uncontrolled reentry - eventually the satellite is low enough and the ambient density is high enough that the vehicle heats, breaks up and is destroyed. The crucial point here is that the *location* of the breakup on the Earth is unpredictable and uncontrolled, in contrast to an impulsive deorbit where the rapid elliptical-orbit descent from a relatively high apogee means that reentry location is determined relatively precisely by the orbital parameters. These Starlink retirements should perhaps be termed `propulsion-assisted orbital decay' - they are more like normal orbital decay but speeded up by the thrusters.
I have also seen `reorbit' for this second use: that's better, but I still don't like it - it sounds like the sat landed and took off again. I prefer to use a handful of extra words and say 'the satellite was moved to a retirement (or graveyard) orbit'. (See Graveyard Orbit).
See also Reentry.
For most box-shaped spacecraft, the two smaller axes are approximately equal in size (i.e. the box has a square cross-section). Rarely, however, all three axes are distinctly different in size. Similarly, a quasi-cylindrical spacecraft might have an elliptical cross section (although I can't think of an example right now). I will use the term 'thickness' for this.
Other sources use the terms height/width/depth to describe the spacecraft main body dimensions. The correspondence is:
|Usual term||GCAT term|
All early dockings involved a docking between two pressurized vehicles, and most had a docking aperture with a hatch system allowing internal transfer of crew or cargo between the vehicles once docked. Use of the term `docking' does not require this: for example, the attachment of the MEV-1 satellite to the Intelsat 901 satellite in 2020 was referred to as a docking, even though the satellites were unpressurized and there were no hatches.
Grapple is used when a robotic arm captures another space object.
Berthing refers to a docking between two objects A and B mediated by a robotic arm. First the robotic arm on object A grapples a fixture on object B. Then the arm is used to move the docking aperture on B next to that on A and joins them together; systems on the docking apertures then complete the connection (just as on a normal docking) and the robotic arm can then ungrapple the fixture on B.
By extension, space (sense 2) is the domain of the subject of astronautics. More quantitatively, the number of satellites and space objects per unit volume is defined over the domain of space (sense 2).
Entry may occur because of a deorbit burn (q.v.), because of natural orbital decay (q.v.), or because the object was already approaching the body on a hyperbolic entry trajectory.
If the object was originally launched from the same body, the term reentry is used to emphasise that the object is returning to its origin. (Actually, the term reentry is occasionally used - sloppily - even if the object has never been to that world before.)
The word `fragmentation' is, however, generally synonmous with `breakup' and implies some kind of disruption of the object, usually with many debris objects resulting.
To be in GEO, you must have a circular (apogee and perigee both close to 35786 km) equatorial (inclination close to 0.0 degrees) orbit.
If the inclination is nonzero, the satellite's ground track will trace out a figure-eight around the equator. If the eccentricity is nonzero, the track will move in an east-west cycle each day. In these cases the orbit is called `geosynchronous' rather than geostationary. If the orbital height is slightly too low or too high, the satellite will drift east or west - hence, a small height adjustment can be used to relocate the satellite.
GCAT's orbital classification defines several sub-cases, but the important distinction is between synchronous (24 hour period) and strictly stationary (circular equatorial as well).
Analogous synchronous orbits are of course possible around other worlds.
The ground station is often more or less a relay point, sending and recieving data from a control center (q.v.) elsewhere - but sometimes it can be colocated with the control center or carry out some of those functions.
Sometimes the ground station is not associated with the satellite owner/operator and is just intercepting data from the satellite, hopefully with the owner's permission. Satellite television receivers and your phone (if it has GPS capability) are examples, although by today's standards we wouldn't really consider them ground stations. Usually we restrict the term ground station to mean a dedicated facility, possibly part of a larger network.
The body-centred radial distance (e.g. geocentric distance) is the more fundamental parameter, but being aware of your satellite's orbital height (and keeping it strictly positive) is important if you are to avoid inadvertent lithobraking (among other reasons).
I characterize the shape of the central object by its length, diameter, and sometimes `minor diameter'. In this context (there is no `up' in space) the length is synonymous with `height', since for most spacecraft and rocket stages the long axis of the object is vertical at the time of launch (when `vertical' still has meaning).
Orbits with inclinations between 62 and 64 degrees and periods from 11.5 to 12.5 hours are a special case, classified as HEO/M (Molniya orbit, q.v.).
The Hill Sphere is also called the `gravitational sphere of influence' of the body. An alternative concept, the Laplace sphere, is also termed a graviational sphere of influence, but I prefer to use the Hill sphere.
For further discusson see the GCAT explanation: Hill Sphere
The orbit is an ellipse or hyperbola which lies in a flat plane, the orbital plane. The orbital plane intersects the coordinate system's equatorial plane in a line called the line of nodes (of course, the line is not well-defined in the degenerate case of zero inclination where the orbital plane and the equatorial plane are the same). Two elements describe the orientation of the orbital plane:
Inclination is zero if you orbit the equator eastwards (same direction Earth turns), between 0 and 90 if you are moving roughly eastwards, 90 for pure polar, and 180 for a westward equatorial orbit.
The longitude in an ICRS or TEME geocentric coordinate system is the right ascension; for a heliocentric ecliptic system it is the ecliptic longitude.
q = a(1-e)
Q = a(1+e)
Hp = q - R
Ha = Q -R
See also Apoapsis.
There are five Lagrange Points in any such system, denoted L1 to L5. Frequently encountered cases have special names: the points in the Sun-Earth system are denoted SEL1 to SEL5, and in the Earth-Moon system EML1 to EML5.
L1, L2 and L3 are on the line joining A and B. L1 is between A and B, L2 is beyond B, and L3 is beyond A. These points are unstable, but a spacecraft can remain there with minimal propellant use.
L4 and L5 lie in the A-B orbital plane and each make an equatorial triangle with A and B. They are formally stable, but in the real universe there are other gravitating objects D, E, F..... which will perturb their positions.
In astronautics, the SEL1 and SEL2 points have been the most used to date. The L3 point rarely comes up in practical situations in astronomy or astronautics, although I think it's vaguely relevant to some of the co-orbital satellites of Saturn.
Lagrange points are sometimes referred to as libration points; this is reasonably correct for L4 and L5, since objects near them librate (oscillate back and forth) around the points, but I don't think it's correct for L1 to L3.
A pad abort (when the engines ignite but the vehicle does not leave the pad) or a pad explosion (when the vehicle does not leave the pad in an intact state) are not considered to be launches.
Launches are broadly characterized as endoatmospheric, suborbital, orbital, or deep-space; we also treat specially orbital launch failures (launches that were intended to be orbital but ended up not being so).
There is no consensus on the exact definition for the start time of air launches. In an air launch, the rocket is dropped from a carrier aircraft, falls for several seconds, and then ignites the first stage rocket motor to begin its ascent. There are three obvious candidates to define the launch time: (1) the aircraft takeoff from the runway, (2) the moment of drop from the aircraft, and (3) the first stage ignition. I adopt (2), while some others adopt (3). (My reasoning is that the ignition of a rocket is not fundamental to putting something in space - one could imagine an EM linear accelerator, or - on a low gravity world - a very big trampoline. So `first motion' is the more extensible definition.)
There are various complications. The main one is the division of responsibilities between the launch vehicle provider and the payload owner, which may differ from mission to mission. In some cases the launch vehicle may deliver the payload to a suborbital trajectory, and an insertion stage considered to be part of the payload completes the rocket firings needed to reach orbit - in this case the launch may be considered a success (as far as launch vehicle reliability is concerned) even if the payload falls in the ocean. Another relevant example is the ZUMA launch in 2018. In that case the launch vehicle entered orbit, but the payload adapter failed to operate. The payload adapter is a device that connects the payload to the launch vehicle; it is usually part of the launch vehicle, but in this case it was operated by the payload owner. The launch vehicle upper stage then performed a deorbit burn, with the payload still attached. This was considered a successful launch for the launch vehicle, although obviously a failed mission for the payload.
If the rocket delivers the payloads to the wrong orbit, a successful mission may still be possible. How far off the orbit can be to allow this depends on the payload.
In 2012 I introduced the launch success fraction definition that is now used in GCAT. See GCAT's section on Launch Success Fractions for details.
For further details see the Space Launch Origins Catalog.
At the present time all launch vehicles are rockets, although gun launch systems have been used for suborbital launches in the past and other possibilities have been envisaged.
See also Rocket (sense 3).
I haven't been entirely rigorous about defining the difference between a main body and an appendage; for example, antennae that are closely packed on the end of the main body are sometimes considered to be part of it. The idea is that the product of length and diameter give an approximate minimum (lower limit) cross section for the space object (with an upper limit provided from the span, q.v.).
See also Diameter and Span.
Note that sun-synchronous orbit (SSO; GCAT orbital categories LEO/S and LLEO/S) is a subset of LEO.
The reader should be very careful about the interpretation of `micro-gravity' and `zero gravity'. In accordance with general relativity, choosing an accelerated coordinate frame whose origin is the center of gravity of the freely falling space object allows one to have zero gravity at that point (gravity and acceleration are fungible *at a point* by change of coordinate frame). But this coordinate frame is not an inertial one, and counter-intuitive things (apparent ficitious forces) will happen when you are not at the centre of gravity.
In particular, do not be fooled into thinking that the lack of gravity is because you are `too far' from Earth and that the gravity is `weak' there. In an inertial coordinate system or in a geocentric coordinate system rotating with the Earth, the strength of gravity on (for definiteness, let us pick:) the ISS is only 13 percent less than that on the Earth's surface. In other words, the acceleration due to gravity is 8.67 meters per second squared instead of 9.81, and you weigh only 13 percent less than you did on the ground. The apparent lack of gravity is exactly the same as that experienced in a falling elevator, or by jumping off a tall building: you will be `weightless' and feel no gravity in your personal coordinate frame - until the moment you hit the ground, which will not be friendly.
There are millions of minor planets in the solar system; ones with well determined orbits are given numbers (`numbered minor planets') and are eligible to be named by the discoverer (or failing that by the IAU Minor Planet Center). I will note that numbered minor planet (4589) was named McDowell in 1993.
Although usually when we talk about ballistic missiles we mean weapons (Sense 3), sometimes the term is used for related vehicles used for test, research or even space launch (sense 2). (It was common in the 1960s to talk about the Atlas first stage of the Atlas Centaur rocket as `the missile', even when it had been custom-built for a space launch vehicle.)
See also Rocket. When talking about `rockets and missiles' (a common juxtaposition), some writers mean `rockets (small unguided projectile weapons) and missiles (larger but still small guided weapons)', e.g. MLRS M26 vs Sidewinder, while others mean `rockets (huge space launch vehicles) and missiles (smaller but still huge ballistic missile weapons)', e.g. Saturn V vs Minuteman 3. Caveat lector.
One effect of Earth's oblateness is that an elliptical orbit rotates slowly in the orbital plane, so that if the perigee starts off at southern latitudes it will at a later time be in northern ones. The rate of this rotation is proportional to a function which is zero if (sin i) squared is 0.8, corresponding to i = 63.43 degrees. Satellites with this inclination will have the latitude of perigee locked instead of rotating, useful for a number of applications.
The jokey use of `norminal' as a replacement for `nominal' in this context appears to have originated as a slip of the tongue by highly-respected SpaceX engineer John Insprucker during a launch commentary. I fully support and endorse the widespread usage of `norminal'.
In some cases the ambient gravitational field is dominated by a single pointlike massive body (or, deviations from this ideal are small enough to be neglected). In this case the orbit of an object is a conic section: an ellipse or a hyperbola. (I will omit discussion of the rare special cases of a parabola and a rectilinear orbit, although we do use parabolic orbits as an approximation to the trajectory of some comets.) Further, the motion of the object along this conic section is described by Kepler's laws. Such an orbit is called a Keplerian orbit. It is described by seven parameters - the six classical Keplerian elements (q.v., also known as orbital elements) and the epoch (time) at which they apply. In a perfect Keplerian orbit, six of these parameters are constant and one (the mean anomaly) increases linearly with time; the elements at one epoch describe the motion of the object at all past and future times.
When the orbit isn't perfectly Keplerian, deviations are often small enough that we can approximate it as a Keplerian orbit whose orbital elements change with time. At any moment, for a given central body mass, the state vector (position and velocity at a given time) is sufficient to define a unique set of Keplerian orbital elements - the orbit the object would have if all forces other than the central body were suddenly switched off. These elements are known as the osculating elements with respect to that central body at the given epoch.
When we consider an object to be in a gravitational field dominated by a single body, we say that it is `in orbit around' that body, even if we are not explicitly calculating its orbital elements. This usage is valid even if the orbit is hyperbolic, or if the object's orbit intersects the body surface.
One of the orbital elements is the eccentricity. If the eccentricity is less than one, the orbit is elliptical; we also say that it is `bound' (remains a finite distance from the central body). If e is more than one, the orbit is a hyperbola, and we can also say it is `unbound' (the object will head off to infinity).
However, note that even if the object is suborbital or escaping, the example sentence would still be valid using sense 1. See further discussion in GCAT's section Space and Orbit.
My personal, prescriptive, opinion is that `on orbit' is a misuse of the preposition `on', makes no grammatical sense, and should be eschewed. Use `in orbit' instead in all cases. Nevertheless I describe here the nuance between `in orbit' and `on orbit' that I observe in its usage.
Examples: "the satellite has been in orbit for three years"; "the satellite deployed its antenna once on orbit". "On orbit" is used to indicate "the state of being in orbit", "the operational phase of orbital operations" (as opposed, for example, to "during launch"). I don't think you would say "the Moon is on orbit", or talk about debris objects being on orbit - the phrase implies not only the dynamical state of being in orbit, but also a state of activity.
For an elliptical orbit, the vast majority of the drag is experienced at periapsis, where the atmosphere is densest. Drag has the effect of slowing down the satellite, reducing the periapsis velocity. This has the consequence that the next apoapsis is lower than it would have been, and thus the semimajor axis is reduced. The counter-intuitive result is that the average velocity of the space object is increased rather than decreased (because the lower semimajor axis has a shorter orbital period and a higher Keplerian velocity).
Another effect of the drag is to make the orbit more circular (the periapsis stays about the same while the apoapsis shrinks). The orbit continues to shrink while circular, until the height is low enough and atmospheric density high enough that the satellite heats up sufficiently to break up and melt: see Entry.
In contrast to active deorbit (see Deorbit) where the final orbit is relatively elliptical and the entry point is therefore easy to accurately predict, the circular final orbit of a satellite undergoing orbital decay makes the location of reentry uncertain. A small change in atmospheric density (e.g. caused by solar activity) or in the orientation of the satellite (changing its effective ballistic coefficient and thus how much drag affects it) can change the reentry time by hours or days. Even on the day of reentry, an uncertainty of many hours is common. During that period the space object circles the Earth several times. Thus, the *location* of reentry is completely unpredictable, the uncertainty region covering multiple continents and oceans - possibly even after reentry has occurred. Sometimes the breakup is detected by infrared sensors on satellites, but on other occasions all we know is that it was seen by radars on one orbit and not on another pass a few orbits later, with reentry having occurred somewhere in between.
A rocket stage may contain instrumentation that sends data to the ground during the first few hours after launch, but this is not usually considered a payload (except if the instrumentation is integrated as a separate package to qualify new technology of some kind).
A special case is that of passive calibration satellites (radar calibration spheres, Lageos, etc) which are considered payloads despite being inert, because they are the `purpose' of the launch. Similarly, ballast dummy payloads on test launches of new launch vehicles are counted as payloads.
Usually in my work, I use `payload' in sense 1: a satellite that isn't (or wasn't at one point) space junk.
For more details, see the definition of Phases in GCAT.
Some space launches are carried out from mobile structures which don't have a fixed geographical location. I thus distinguish between the `platform' (the vehicle used as a launch platform) and the `site' (the geographical location at which the platform was located when the rocket separated from it).
Platforms which have been used to launch rockets include surface ships, submarines and aircraft. (I also include the KSC pad 39 mobile launch platforms, just because I want to record which one was used for which launch and it's a useful database field to stash that particular information.)
See also Launch Origin, and see the discussion in GCAT on Launch Platforms.
Sometimes, however, the fuel contains its own oxidizer; it is a `monopropellant'. Often, as with the monopropellant hydrazine, a catalyst allows the reaction to proceed.
In this sense, we also use `range rate' to mean the rate of change of the range, i.e. the radial component of its velocity relative to you. Positive rates mean the object is getting further away.
For example, the Eastern Test Range in the early 1960s included the Cape Canaveral launch site and stations on Caribbean islands such as Grand Turk and Antigua, out to the Ascension Island tracking station and missile impact zone in the middle of the South Atlantic.
Associated with this sense are several additional terms:
The term reentry vehicle (RV) is particularly associated, however, with ballistic missiles. In an operational ballistic missile weapon, the RV contains an explosive warhead (in some cases a nuclear weapon). However, in test launches the warhead is replaced by test instrumentation: for example, when the US test-launches a Minuteman missile from Vandenberg to Kwajalein, or North Korea tests a Hwasong 15 flying it over Japan into the Pacific, the reentry vehicles do not contain warheads. (I emphasize this because the terms reentry vehicle and warhead are often conflated and confused in popular descriptions of missiles).
Sense 3 is the most common usage today.
Similar but less common expressions include IOBM (In Ocean By Mistake) and SWLC (Salt Water Leak Check). Also known as `having a bad day'.
Spacecraft beyond Earth orbit, including artificial satellites of the Sun and of other worlds (Moon, Mars, etc.) are usually called space probes rather than satellites. The phrases `satellites and space probes' is often used in the sense of `artificial satellites of the Earth and interplanetary spacecraft, respectively'.
When I use the word satellite, I usually mean Sense 4 or, occasionally, Sense 5. However instead of Sense 5 I prefer to use the word `payload'. I never mean Sense 7. Senses 3 and 4 are both in wide use and are quite different; therefore, you should be careful to be clear which one you mean (e.g. in questions like `how many satellites are in orbit?')
See also Small satellite
|Minisatellite||100 to 500 kg|
|Microsatellite||10 to 100 kg|
|Nanosatellite||1 to 10 kg|
|Picosatellite||0.1 to 1 kg|
|Femtosatellite||Less than 0.1 kg.|
Note that despite the use of SI prefixes, the words micro, nano, pico and femto do not have their usual (3-orders-of-magnitude) SI meanings.
For airless worlds it's a bit tricky: if you are standing on the lunar surface, are you in space? What if you jump a few centimetres, are you in space then? I like to think of a future lunar space traffic control distinguishing between local traffic and long range traffic at some altitude like 1 km, but I'd be interested to hear other suggestions.
I will sometimes specify `outer space' to distinguish from other English usages of the word (`inner' psychological space, living and personal space, hit the space (ASCII 0x20) key...) as well as mathematical ones (vector space, topological space, parameter space).
To date, space stations have been launched without anyone aboard, but I don't think that's a requirement of the definition as long as other crews are meant to arrive later.
The docking of Gemini ships with Agena target vehicles could be seen as the creation of a temporary space station. The Agena did not have a pressurized interior, however, and we do not usually regard the Gemini/TDA/Agena complex as a space station.
The docking of Soyuz 4 with Soyuz 5, with the transfer of crew between the two, is also close to being a temporary station but is not regarded as such. The Apollo CSM/LM complex is equivalent.
DOS 1 (Salyut 1) is regarded as the first space station. It was launched without a crew. Two Soyuz crew ferry spaceships docked with the station, although only one succeeded in entering it and living aboard it for a period of time.
Almaz 1 (Salyut 2) and DOS 3 (Kosmos-557) are regarded as space stations, even though they both failed before crews could be sent to them.
Skylab was the first space station which was successfully boarded by multiple visiting crews, which arrived and departed in Apollo CSM spaceships.
Space stations may be made up of multiple modules assembled in orbit (but are not required to be). The difference between a docked module and a visting cargo ship is fuzzy; Kosmos-1267 was docked to Salyut 6 and remained attached for the rest of its life. One can regard it as a large cargo ship or as part of the first two-module station. Salyut-7 also had temporary attached modules, but Mir was the first station with modules designed to be permanent parts of the station.
Some writers assert that stations have to be large, and that smaller examples such as DOS 1 and China's Tiangong 1 should be instead called `space laboratories' or spacelabs, rather than full space stations. I reject this interpretation.
The NASA/ESA Spacelab was a space laboratory carried within the Shuttle payload bay. As it did not fly separately from the spaceship it was aboard, and could not receive multiple ferry spaceships, it does not meet the definition of a space station.
Several organizations have proposed to develop `human-tended' space laboratories which would not normally carry a crew but which could be visited by a crew for maintenance. Typical proposals of this kind would meet the definition of space station as laid out above, including having a pressurized interior section, and I would consider them as such. In contrast, the Hubble Space Telescope, although human-tended, has no pressurized cabin.
Note that ground-based weapons, such as powerful radio jammers or lasers, may also damage space objects. They are not `space weapons' by my definition. That does not make them less bad.
Also note that self-destruct mechanisms, as used on many Soviet satellites, cause damage to the host space object and may generate space debris that accidentally damages other space objects, but are not space weapons by this definition. Further note that the definition involves intent, which is notoriously hard to determine.
A hand gun carried by an astronaut which could be used to harm another astronaut is a space weapon by this definition. However, we are usually interested in weapons that are designed to destroy free flying space objects: these are also called antisatellite weapons (ASATs). ASAT systems that have been developed include:
|Type of ASAT||Description||Examples|
|Co-orbital with warhead||Orbital rendezvous or flyby followed by explosion||IS (USSR)|
|Suborbital nuclear||Flyby with explosion||Nike-Zeus 505, Thor 437 (USA)|
|Suborbital kinetic||Physical intercept at high speed||F-15/MHV, Aegis SM-3 (USA), DF-21 ASAT (China), Nudol (Russia), PDV (India)|
The other kind of space weapon is that aimed at targets on the ground. By my definition of space object (sense 1) this includes any weapon that travels through space on its way to the target, including live-warhead ballistic missiles (MRBM, IRBM, ICBM). (One might argue that tests with dummy warheads are tests of a space weapons *system* but are not in themselves space weapons.)
Less controversially, it includes weapons that are based in space. Laser battle stations proposed by the US during the Reagan Administration would fall in this category. Weapons temporarily in orbit are also included. Tests of such a weapon were carried out by the USSR from 1966 to 1971: the OGCh (Orbital Payload) of the R-36O missile, known in the West as FOBS (Fractional Orbital Bombardment System), which completed one or slightly less than one orbit and delivered its reentry vehicle and dummy warhead to a test range target. No live warheads were carried.
Under the Outer Space Treaty, no weapons are allowed on the Moon or other planets. Weapons are however allowed in orbit, except for weapons of mass destruction, including nuclear weapons - they are (since 1967) not allowed in space at all.
The term `space vehicle' is also used,.
The various space agencies track the duration of spacewalks, but use different definitions to do so. To carry out a spacewalk, a typical sequence involves depressurizing the airlock (DP) (marked at some specified pressure value); removing spacesuits from spaceship power to internal battery power (BP), airlock hatch opening (HO), egress (EG, i.e. physically emerging from the airlock), and then at the end of the spacewalk ingress (IG), hatch close (HC), suits to spaceship power (SP), and airlock repressurization start (RP) and end (RPE). The steps are not necessarily in the same order for different space programs. Example definitions are:
|Agency/Program||Duration of spacewalk|
|NASA/Apollo||RP - DP (3.5 psi)|
|NASA/STS,ISS||RP - BP|
|Roskosmos||HC - HO|
|My records: RP - DP (0.7 psi).|
If you want to compare spacewalk records across programs you should use a consistent definition to do so, as the different methods can easily give durations that differ by 5 minutes or more. That's why I've gone to the trouble of estimating durations for all spacewalks by my own criterion.
During the Gemini program, some spacewalks involved DP/RP and HO/HC but no complete EG/IG (that is depressurization and hatch opening but no egress) - these were called SEVAs (Stand-Up EVAs). During the Mir and ISS programs, Roskosmos carried out `spacewalks' with DP/RP but no single HO/HC and no EG/IG; they depressurized the transfer compartment of the station and then relocated hatches from one docking port to another without leaving the compartment. Whether these have been considered as spaceawalks varies; there are other similar examples.
For that reason I refer to and keep track of `depress activities' (i.e. periods when the crew were in a pressurized spacesuit but not in any other pressurized environment) as a more general concept. I don't like the name but haven't been able to come up with a better one yet: the intent is to emphasize that it's the vacuum that matters (because that's what will kill you). The issue of whether you are inside or outside seems a secondary one to me. The issue of whether your suit is independently powered seems even less important; you can imagine a solar powered suit which was always on battery power, or a brief spacewalk in a suit that was entirely unpowered, or spending long periods in a self-powered suit entirely within the host spaceship. My choice of 50 mbar (0.7 psi) as the 'almost vacuum' pressure value used to mark the spacewalk start and end time is admittedly arbitrary - I feel that consistency is the most important thing rather than the exact choice.
See also length, diameter.
Nevertheless, there is overhead associated with each stage, and stage separation is a tricky thing that has caused many failures. Modern launch vehicles tend to have only two stages. In the past, some rockets have had five or more.
The Isp plays a key role in the rocket equation. If a rocket vehicle has total initial mass M and burns a mass m of propellant, giving it final mass M-m, the change in velocity is dV = Ve ln( M/ (M-m))
Isp is typically in the range 200 to 480 seconds for chemical rockets and several thousand seconds for electric propulsion (ion engines).
In modern times there can be organizations from many countries involved in a space launch; the launch site, launch vehicle manufacturer, launch vehicle operator, satellite manufacturer, and satellite owner may all be from different states. There is no consistent practice defining which of these should report registration of the satellite to the UN.
In GCAT, I record the state of registration separately from the state of the owner - they are often the same, but by no means always.
Note that there is some legal disagreement about whether the state of registration of a satellite can change after launch, for example because the satellite is sold to another owner.
Usually the vectors are given in Cartesian coordinates with the central body at the origin.
If you orbit a spherical world, your orbital plane stays fixed in inertial space (the longitude of ascending node is constant, see Keplerian Elements). Let us take as an example a satellite orbiting the Earth as it in turn orbits the Sun, but we'll pretend the Earth is spherical instead of oblate.
Then suppose you launch in January into a `noon-midnight' orbit with almost polar inclination which passes over the centre of the Earth's day side and the center of the night side. The Earth-Sun line passes through your orbit plane, and your orbit normal (the vector perpendicular to the orbit plane) is perpendicular to the Earth-Sun line.
Now wait 3 months to April: the Earth has moved 90 degrees around its orbit. The orbit plane is pointing in the same direction in space, but the Sun is in a totally different direction (if it was right in front of you before, now it is at your left hand). The orbit plane is now (depending on the orbit inclination) more or less perpendicular to the Earth-sun line and your satellite is now passing over the terminator: regions of the Earth which are in dawn or dusk.
3 months further on, and 180 deg round the Sun from the start, you're back to noon-midnight.
However, now lets squish the Earth at the poles to its actual oblateness. This causes orbital precession: in particular the orbit longitude of ascending node changes with time. This potentially annoying effect can be turned to good use: the rate of node change depends on height and inclination. At a given height there is a particular inclination (beween 95 and 101 degrees for heights in the LEO range) at which the node change exactly cancels out the change in the direction of the Sun as seen from the Earth in inertial coordinates So if you're in this orbit the orbit plane tracks the direction of the Sun, and a noon-midnight orbit remains a noon-midnight orbit. This means that shadows stay the same length (good for spy sats), you can watch storms forming in the Atlantic at the same time each day (good for weather sats), and that you can pick an orbit where the Sun never gets in your way when you are looking out into space (good for astronomy satellites). Super useful!
These UTC times are presented in one of two representations:
Some deep space missions follow the egregious practice of reporting events in so-called `Earth received time' (ERT). They will say, for example, `Opportunity landed on Mars at 0505 UTC', when they actually mean `The radio signal Opportunity sent when it landed on Mars reached Earth at 0505 UTC', but the actual landing occurred at 0454 UTC - these are two different events, not two time representations of the same event. This practice is particularly heinous when they fail to specify which convention is being used. You will not find ERT in this work.
For space launches, `inertial velocity' usually means 'velocity relative to the GCRS'; the ECEF rotates relative to the GCRS once a sidereal day.
The Earth orbits the Sun at 30 km/s (i.e. roughly speaking its ICRS velocity is 30 km/s and the Sun's is 0.015 km/s). An object at rest relative to the Sun will then have an Earth-relative velocity that varies from -30 km/s to +30 km/s just becuse of the Earth's motion during one year. Because of this, 'what is the fastest space probe?' is a very problematic question, as the questioner often is thinking they mean '.. in the GCRS frame', and the answer is just not very useful because of the huge component just due to where the Earth is in its orbit at a given moment.
Some rockets (the Atlas sustainer, and the Chang Zheng 2 second stage) have main engine cutoff (MECO) followed by a sustained period of vernier engine firings followed by the final vernier engine cutoff (VECO) prior to stage separation.
Although a rigorous geophysical definition of a world would have a size boundary depending on its composition, in practice I treat all nonstellar objects with radius greater than 200 km as worlds.